Cross-slit silicone valves are widely used for this purpose: an inwardly-convex silicone rubber dome with a cross-shaped slit is mounted in the outlet opening. Under dispensing pressure it inverts (bulges outwardly) and the slits open up for flow. When pressure is relieved the elastomer dome flips rapidly back to its original form, closing the slits. Flow is cut off, and on recovery compensation air can enter (with some product suck-back) because little pressure difference is needed to open the slits for some reverse air flow. These slit silicone valves work well, but the silicone elastomer is expensive, and neither recyclable nor degradable.